1. Field of the Invention
The present invention relates to the removal of nitrogen oxides from waste gases.
2. Description of the Prior Art
A substantial amount of research has been directed at efforts to eliminate or reduce the nitrogen oxide [NO.sub.x ] content of waste gases emanating from boilers, engines, turbines and other systems which incorporate combustion processes involving the introduction therein of air consisting of nitrogen (N.sub.2) and oxygen (O.sub.2). Products of these combustion processes include nitrogen, carbon dioxide (CO.sub.2), carbon monoxide (CO), water (H.sub.2 O), unburned oxygen, unburned fuel and NO.sub.x.
The oxides of nitrogen are particularly undesirable in exhaust effluents because of their highly corrosive nature. They contribute greatly to "acid rain" and air pollution problems.
Efforts to reduce or eliminate NO.sub.x from waste gases can be categorized into two types of techniques. The first involves modification of the combustion process itself to reduce the amount of NO.sub.x produced.
The second type involves treatment of the waste gas itself to either remove the NO.sub.x from the waste gas effluent or to convert it to a less noxious form.
Examples of the latter technique include the catalytic reduction of NO.sub.x to nitrogen utilizing reducing agents such as methane, ammonia, etc. U.S. Pat. Nos. 3,846,981; 3,826,810; 3,449,063; 3,279,884; 2,975,025; 3,232,885 and Canadian Patents 668,384 and 787,836 are exemplary of the prior art describing these techniques. They suffer from the disadvantage that they require expensive and sensitive catalysts which are subject to poisoning and destruction at the temperatures generated by the reduction processes.
An example of the former type of NO.sub.x control, i.e., modification of the combustion process itself, is the so-called "fuel-staging" or "gas-reburning" techniques which involve introduction of methane into the fuel itself. The combustion process produces hydrocarbon fragments or radicals which can reduce NO.sub.x to N.sub.2. The disadvantages of fuel staging are that the methane must be injected into the combustion zone and that most of the methane is consumed in reactions with oxygen.
The reactions in fuel staging which lead to the reduction of NO.sub.x to N.sub.2 are as follows: EQU CH+NO.fwdarw.HCN+O EQU CH.sub.2 +NO.fwdarw.HCN+OH EQU HCN+OH.fwdarw.HNCO+H EQU HNCO+H.fwdarw.NH.sub.2 +CO EQU HNCO+H.fwdarw.NCO+H.sub.2 EQU NCO+H.fwdarw.NH+CO EQU NO+NH.sub.2 .fwdarw.N.sub.2 +H.sub.2 O EQU NO+NH.fwdarw.N.sub.2 +OH
It can be seen that the success of such a NO reduction pathway depends on the generation of an adequate supply of CH and CH.sub.2 radicals. These radicals are supplied by the "reburning gas", which is generally methane, due to its availability and low price. In practice it has been found that in order to achieve adequate NO reduction, as much as 20% of the total combustion heat release must be supplied by the injected methane [Bartok et al, "Control of NO.sub.x by Fuel Staging", Proceedings of the 1987 Joint Symposium on Stationary Combustion NO.sub.x Control, March, 1987]. An examination of the reaction pathways of C.sub.1 and C.sub.2 hydrocarbons can demonstrate why this is the case. The reactions of methane with oxygen lead to CO through reactions of the type: EQU CH.sub.4 +O.fwdarw.CH.sub.3 +OH EQU CH.sub.3 +O.fwdarw.CH.sub.2 O+H EQU CH.sub.2 O+.fwdarw.CHO+OH EQU CHO+O.fwdarw.CO+OH
Thus, reactions with oxygen do not yield the CH and CH.sub.2 radicals necessary for NO reduction. The only reactions leading to these radicals are those involving hydrogen which produce acetylene as a precursor to CH and CH.sub.2. It is thus clear that when methane is simply burned, a large percentage of it follows a pathway to CO which does not produce CH or CH.sub.2 radicals as intermediate products.
One way to improve the utilization of the reburn gas is to directly utilize acetylene. The fact that acetylene directly reacts with nitric oxide was demonstrated by Q. Le and M. Vanpee ["Free Radical Concentration Measurements in Nitric Oxide--Acetylene Flames", Combustion and Flame, Vol. 62, pp. 193-210 (1985)]who stabilized a C.sub.2 H.sub.2 --NO low-pressure burner. The stoichiometric balance for this reaction was 17% C.sub.2 H.sub.2 and 83% NO. Richer fuel burns were also considered (27% C.sub.2 H.sub.2 and 73% NO). Their flames produced long-lived free radicals of OH, CN, C.sub.2, CH, and NH.
Parker and Wolfard [Fourth Symposium on Combustion, The Combustion Institute, pp. 420-428 (1952)]investigated the characteristics of flames supported by NO and NO.sub.2. The burning velocities of different hydrocarbons with NO and NO.sub.2 were measured and compared. The burning velocity is indicative of the relative reaction rates for each burn. It was found that the acetylene reactions have the highest velocities for all of the hydrocarbon reactions with NO and NO.sub.2. In addition, the acetylene reactions have the smallest quenching diameters which suggests that the temperature at which the reaction begins in the flame front is very high. Therefore, contrary to general belief, flames are readily obtained with NO provided the conditions take into account the large quenching diameters and high ignition energies which are involved.
Experiments were conducted by Shaub and Bauer ["The Reduction of Nitric Oxide During the Combustion of Hydrocarbons: Methodology for A Rational Mechanism", Combustion and Flame, Vol. 32, pp. 35-55 (1978)] with shock tubes to study the reaction of NO with acetylene and other hydrocarbons. They found that CH and CH.sub.2 radicals are important radical intermediates in the destruction of NO with acetylene.
These studies indicate that acetylene can be utilized for NO.sub.x emission control. The reburning can be conducted near the main heat release zone, or in the exhaust duct with the reheating facilitated with an atmospheric plasma source. Furthermore, it has been shown that acetylene is more effective than methane in destroying nitrogen oxides.
A disadvantage in using acetylene in reburning instead of methane is that acetylene can only be obtained in small steel cylinders of 300 liter volume, because of its inherent volatility. To obtain large quantities of the gas, many such steel cylinders are required which keeps the price of the gas very high, and not commercially attractive. This is why acetylene is primarily used for a cutting and welding torch fuel, and not for combustion applications.
It is an object of the present invention to provide a method for the removal of nitrogen oxides from waste gases which is not subject to the abovenoted disadvantages.